This proposal is designed to provide insights into the process whereby the circulating clotting factor precursor, native factor XI, is converted to the active procoagulant, activated factor XI (XIa). The properties and composition of native factor XI will be studied; native factor XI will be activated in an isolated system; and the properties and composition of the activated form will be compared to those of the precursor. Parameters to be investigated include size/shape, electrophoretic mobility, sedimentation coefficient, enzymatic activity, adsorption properties, subunits, N-terminal amino acids, amino acid composition.